To improve the performance of a process tools, a metrology module is typically employed to measure processing parameters on the substrate after the substrate has been processed. If one or more of the process parameters are outside an acceptable tolerance range, the substrate is reprocessed or rejected. Moreover, the process tool may be adjusted to avoid faulty processing of subsequent substrates.
One of the requirements of the metrology module is that it does not degrade the throughput capability of the process tool. In general, to improve throughput, it is desirable for the measurement speed to be as fast as possible favoring less measurement locations on each sample or only measuring a fraction of the total number of samples being processed. However, to improve the probability of detecting and analyzing a problem with the process tool, a large number of measurement locations and all of the processed samples should be measured. Thus, a balance is typically struck between throughput and sampling rate.
Once the metrology measurement is made, the data can be used two different ways. In the passive mode, the metrology data is analyzed to see if it is within the acceptable tolerance range of the process tool. If it is, no further action is taken and the process tool continues processing subsequent substrates. The engineer may also choose to slightly modify the process parameters if, for instance, a small drift is observed within the acceptable tolerance range. If the data is not within the acceptable tolerance range, however, this information is provided to the engineer and/or used to stop the processing of subsequent substrates.
In the active mode, the metrology data is analyzed in the same manner. If the data indicates the process is well centered in the tolerance range, no further action is taken. However, if the data indicates that the process is skewed from the center of the tolerance range but within the tolerance range, some parameter associated with the process may be modified to attempt to center the one or more parameters being measured. If the data indicates that the response is not within the tolerance range, this information is used to alert the engineer and/or stop the processing of subsequent substrates.
Conventionally, measurements of all important parameters related to the processing of the substrate are made on a designated number of processed substrates at a designated number of locations. To increase throughput, less than all of the processed substrates or less locations on a substrate are typically measured, which unfortunately increases the risk of not detecting problems associated with the processing tool. For example, every fifth wafer could be measured for two parameters at five sites on the wafer to not degrade the throughput of the process tool. The engineer's choice of measuring frequency and number of locations per substrate can vary tremendously based on numerous parameters. Thus, what is needed is an enhancement to the throughput of the metrology module to increase the sampling rate of the number of substrates and the number of sites per substrate.